Configuration measuring apparatus and method

ABSTRACT

Arranged in both sides of the thin plate are optical displacement gauges that irradiate measurement lights to surfaces of a thin plate and receive the measurement lights reflected by the surfaces so as to measure displacements of the surfaces of the thin plate. Variation of thickness of the thin plate is obtained on the basis of the displacements of the surfaces of the thin plate measured by each of the optical displacement gauges. Each of the optical displacement gauges detects the displacement of the surface of the thin plate with high accuracy by irradiating the measurement light to the thin plate two times.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a configuration measuring apparatus and method, and particularly to a apparatus and method for measuring a surface shape of a thin plate member such as a wafer for manufacturing a semiconductor, in which a little shape variation is required in a surface direction.

[0002] The wafer for manufacturing the semiconductor is constituted by a thin plate member such as a silicone or the like. In order to form a semiconductor device or a circuit on a wafer surface, a photo engraving technique, a printing technique, various kinds of micro-fabrication techniques or the like is applied.

[0003] In the wafer to which such working process is applied, it is important to increase a flatness of a surface. When the flatness of the wafer is deteriorated, a pattern of the device or the circuit is unclearly formed, or a profile of a material to be printed on the wafer surface in a pattern shape becomes indefinite, at a time of photo engraving. In particular, as a densification or a large-size of the semiconductor device or the circuit is promoted, the problem mentioned above becomes significant.

[0004] In a semiconductor manufacturing step, various kinds of processes are frequently executed in a state in which a whole surface of the wafer is supported in a closely contact manner to a flat supporting surface by a means such as a vacuum adsorption or the like. At this time, in the case that a thickness of the wafer has a dispersion, the dispersion of the thickness appears as a dispersion of the flatness of the wafer surface as it is at a time of supporting the wafer to the flat supporting surface in a closely contact manner.

[0005] Accordingly, it is required that the dispersion or the variation in correspondence to the place is not generated in the thickness of the wafer. In order to estimate whether or not the thickness variation of the manufactured wafer is large in the manufacturing step of the wafer or the like, it is necessary to accurately and efficiently measure the thickness variation of the wafer.

[0006] As a conventional wafer thickness variation measuring apparatus, there is a technique described in Japanese Patent Application Laid-Open No. 2000-283728. In this technique, a displacement of the wafer surfaces with respect to optical sensors arranged in side portions of both surfaces of a disc-like wafer while rotating the wafer in a perpendicularly standing state, whereby a magnitude of the thickness variation of the wafer is calculated from the displacement of the wafer surfaces measured by the right and left sensors. Scanning the wafer in a radial direction thereof by the optical sensors attains measurement of the thickness variation with respect to the whole surfaces of the wafer.

[0007] The conventional wafer thickness variation measuring apparatus mentioned above has a limit in accuracy of measuring the thickness variation, so that the apparatus can not achieve the measuring of the thickness variation with high accuracy required for manufacturing a semiconductor device or a circuit having high accuracy and density in the future.

[0008] In an optical displacement gauge used in the measuring apparatus, a change of distance between the sensor and the wafer generates a interference signal of sine wave shape with a cycle that is one half of a wavelength λ of a laser beam. In order to keep a length standard and a traceability regulated in ISO, a frequency stabilized He—Ne laser having a wavelength of about 633 nm is used as the laser beam for measurement. In order to execute the measurement in accordance with the method mentioned above, an intensity of the sine-waved interference signal is detected in an analog manner, and a distance within the wavelength of λ/2 is detected by a computation.

[0009] However, since the sine-waved interference signal is generated by utilizing a polarization property of the light, the interference signal has a slight sift with respect to an ideal sine-waved waveform due to a significantly little polarized light leak or the like in an optical part such as a λ/4 wavelength plate or the like used in the conventional optical displacement gauge, thereby generating an error in measuring a distance.

[0010] In correspondence to that the density of he semiconductor device or the circuit has become higher recently, a measurement of the wafer thickness variation at an accuracy not less than 0.0015 μm will be required in the near future. Owing to the measurement-principle thereof, such highly accurate thickness variation measurement mentioned above is hardly possible. In the conventional optical sensor, an accuracy of about 0.003 μm is a limit.

[0011] Besides the wafer for manufacturing the semiconductor device, there are technical fields in which a significantly high accuracy in measurement of the thickness variation is required, such as a substrate for magnetic disc.

[0012] Therefore, an object of the present invention is to attain measurement of a thickness variation of a thin plate such as a wafer with high accuracy and high efficiency.

SUMMARY OF THE INVENTION

[0013] In a configuration measuring apparatus and method in accordance with the present invention, arranged in both sides of the thin plate are optical displacement gauges that irradiate measurement lights to surfaces of a thin plate and receive the measurement lights reflected by the surfaces so as to measure displacements of the surfaces of the thin plate. Variation of thickness of the thin plate is obtained on the basis of the displacements of the surfaces of the thin plate measured by each of the optical displacement gauges. Further, in the apparatus and method in accordance with the present invention, each of the optical displacement gauges detects the displacement of the surface of the thin plate with high accuracy by irradiating the measurement light to the thin plate two times.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Further objects and advantages of the present invention will become clear from the following description taking in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which:

[0015]FIG. 1 is a perspective view of a whole of a configuration measuring apparatus;

[0016]FIG. 2 is a detailed structural view of a main portion of a configuration measuring apparatus in accordance with a first embodiment of the present invention;

[0017]FIG. 3 is a schematic view illustrating a measuring operation of a wafer;

[0018]FIG. 4A is a graph showing an operation with time of the wafer;

[0019]FIG. 4B is a graph showing an operation with time of the optical displacement gauge;

[0020]FIG. 5 is a perspective view of a sensing pin reflector;

[0021]FIG. 6 is a side elevational view of the sensing pin reflector;

[0022]FIG. 7 is a bottom elevational view of the sensing pin reflector;

[0023]FIG. 8 is a detailed structural view of a main portion of a configuration measuring apparatus in accordance with a second embodiment of the present invention; and

[0024]FIG. 9 is a detailed structural view of a main portion of a configuration measuring apparatus in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025]FIG. 1 is a schematic view of a configuration measuring apparatus, in particular shows a thickness variation measuring apparatus for a semiconductor wafer. A wafer w is held by a ring-like hollow spindle 100 in a state of being vertically stood, and rotated within a vertical surface by a rotational drive of the hollow spindle 100. Optical displacement gauges 200 are respectively arranged in side portions of both surfaces of the wafer w. Although only a front side of the apparatus is shown in the drawing, the optical displacement gauge 200 is arranged at a symmetrical position in a back side to the front side. A pair of optical displacement gauges 200 are mounted so as to freely move in a direction parallel to the surface of the wafer w, and thereby a measured position of displacement by the optical displacement gauges 200 moves right and left on a radius of the wafer w. Specifically, a mounting table 22 of the optical displacement gauge 200 is linearly moved by a rotational drive of a ball screw 24. By combining the rotation of the thin plate member and the movement along the radial direction of the optical displacement gauge, a measurement covering a whole surface of the thin plate member is achieved. The same function can be achieved by an optical system in which an irradiation position of a measurement light onto the thin plate and receiving position of a reflected light are changed without moving a main body of the optical displacement gauge. Such a scanning measurement is suitable for an efficient quality control or the like in a shop-floor.

[0026] By adding the displacements on both surfaces of the thin plate measured by a pair of optical displacement gauges, a variation of thickness of the thin plate can be determined. A thickness variation computing means for executing the processing so as to determine the thickness variation of the thin plate can be constituted by an electronic circuit. The processing procedure is programmable by using a processing apparatus such as a microcomputer or the like.

[0027] The thin plate may be made of various kinds of materials and have various kinds of shape dimensions as far as it requires the measurement of the thickness variation with high accuracy. The thin plate may be made of a conductive material or an insulating material. A material in which a quality of material or an electrical property is different in accordance with a place may be employed. A laminate constituted by a plurality of materials may be employed.

[0028] Specifically, a metal plate, a ceramic plate, a resin plate and the like which constitute a material of a semiconductor wafer magnetic disc such as a silicon or the like may be employed. A shape of the thin plate may employ the other shapes than a circular shape although the wafer or the like is often formed in a circular plate shape or a disc shape. The surface of the thin plate preferably has a fine reflecting property such as a mirror surface or the like.

[0029] In the case of using the sensing pin reflector, the surface of the thin plate may have no reflecting property.

[0030] Used as the optical displacement gauge, there is used a measuring device or apparatus having a function of measuring a distance to a subject to be measured or a distance change so as to measure the displacement of the surface of the subject by irradiating a measurement light onto the subject and receiving the measurement light reflected on the surface of the subject.

[0031]FIG. 2 shows a structure of an optical displacement gauge of a configuration measuring apparatus in accordance with a first embodiment of the present invention. The illustrated structure is for measuring the thin plate principally by a light having a spot diameter of approximately 0.1 mm.

[0032] As shown in FIG. 2, a linear polarization laser beam having a single wavelength emitted from a light output section 301, which has a laser beam stabilized in a frequency so as to have wavelength λ as a light source, is set its polarization surface to oblique 45 degree with respect to the polarization beam splitter 2, and therefore branched into a measurement light and a reference light by the polarization beam splitter 2 after injected into a lens 1. If the laser beam were injected to an optical center of the lens 1, it would be reflected and returned by a thin plate 4 or a mirror 8 along the same path. Thus, the laser beam is injected to a position having an offset larger than a beam diameter with respect to the optical center of the lens 1.

[0033] The measurement light branched by the polarization beam splitter 2 is irradiated onto the thin plate 4 through a λ/4 wavelength plate 3 to be reflected at a first time.

[0034] Then, the light is transmitted along the following path so that the light is again irradiated onto the thin plate 4 for an accurate measurement. The light reflected on the thin plate 4 is again transmitted through the λ/4 wavelength plate 3, thereby its polarization direction is changed at 90 degrees, and the light is again injected into the polarization beam splitter 2 to be reflected in a direction different from the incident direction. The light is collimated by passing through a lens 5. This is because a focal distance of the lens 5 is coincided with a distance from the reflection position of the thin plate 4 to the lens 5. Then, the light is irradiated to a mirror 7 through a λ/4 wavelength plate 6. The light reflected on the mirror 7 is again passed through the λ/4 wavelength plate 6, resulting in a rotation of the polarization surface at 90 degrees. Thereafter, the light is injected to the polarization beam splitter 2 through the lens 5, and is transmitted in a direction different from the incident direction due to the change of its polarization surface. The transmitted light is reflected by the mirror 8 arranged at a position of the focal point, and is again passed through the polarization beam splitter 2. Then, the light is collimated by again passing through the lens 5, and is irradiated to the miller 7 through the λ/4 wavelength plate 6. The light reflected by the miller 7 is again passed through the λ/4 wavelength plate 6 resulting in a rotation of the polarization surface at 90 degrees. Thereafter, the light is injected to the polarization beam splitter 2 through the lens 5, and is reflected in a different direction due to the change of its polarization surface. The reflected light is again irradiated to the same portion in the thin plate 4 through the λ/4 wavelength plate 3, thereby a reflection at a second time is performed.

[0035] Further, in order to introduce to a light-receiving and computing section 302 for measuring the light, the polarization direction is changed by again passing through the λ/4 wavelength plate 3, and the light passes through the polarization beam splitter 2, again passes through the lens 1 and is introduced to the received light computing portion as a measurement light 9. Further, in order to introduce the light to a light-receiving and computing section 302, the light is again passed through the λ/4 wavelength plate 3, so that its polarization direction is changed. Then, the light is again passed through the lens 1, and is introduced to the light-receiving and computing section 302 as a measurement light 9.

[0036] A reference light which is interfered with the measurement light 9 having a phase changed due to a change of the position of the thin plate 4 so as to generate an interference fringe is produced in accordance with the following procedures.

[0037] The reference light reflected in a direction of 90 degrees by the polarization beam splitter 2 so as to be branched is reflected by the mirror 8, and is again injected to the polarization beam splitter 2. Then, the reference light is reflected toward the lens 1, and is introduced to the light-receiving and computing section as a reference light 10.

[0038] The light-receiving and computing section 302 receives a mixed light of the measurement light 9 and the reference light 10, and performs a computing process for computing phase differences so as to measure displacements of the surface of the thin plate.

[0039] In this apparatus, a travel of the measurement light is changed on the basis of a difference of a distance from the optical displacement gauge to the surface of the thin plate 4, whereas a travel of the reference light is constant. Accordingly, the displacement of the surface of the thin plate can be determined by measuring a difference of travel between those of the measurement light and the reference light.

[0040] The measurement light 9 is transmitted to the thin plate surface at two times, and a displacement d of position of the thin plate surface can be accurately detected as a displacement of an optical path of 4d by the light-receiving and computing section 302.

[0041] The light output section 301 produces the reference light and the measurement light with the wavelengths accurately controlled by the laser beam stabilized in frequency. A light branching and mixing portion 304 is constituted by an optical system such as the polarization beam splitter, the λ/4 wavelength plate, the mirror and the like. The light-receving and computing section 302 is constituted by a photoelectric transfer element, a processing circuit for electric signals, an arithmetic operation circuit and the like.

[0042] As a particular structure of the apparatus mentioned above, there can be applied a technique of a three-dimensional configuration measuring apparatus disclosed in Japanese Patent Application Laid-Open No. 3-255907 by the present inventors.

[0043] The lens 1 which converges the output light to supply to the light branching and mixing section 304 can be provided between the light output section 301 and the light branching and mixing section 304.

[0044] The lens 1 functions to focus the measurement light irradiated to the thin plate 4 so that the measurement light is irradiated only to a narrow area of the thin plate 4 for increasing measurement accuracy. By arranging the lens 1 between the light output section 301 and the light branching and mixing section 304 not between the light branching and mixing 304 and the thin plate 4, it is possible to make the travel from the light branching and mixing 304 to the thin plate 4 short so as to reduce an influence of fluctuation of air interposed between the optical displacement gauge and the thin plate. Further, since the distance from the lens to the thin plate can be set long, it is possible to make a spot diameter on the thin plate small to about 0.1 mm, resulting in accurate measurement at an area within 1 mm from an outer periphery of the thin plate.

[0045] Further, a convergent optical system constituted by a focus lens converging the mixed light and supplying it to the light-receiving and computing section 302 may be provided between the light branching and mixing section 304 and the light-receiving and computing section 302. The convergent optical system is constituted by the optical members such as the lens, the mirror and the like. The convergent optical system improves an accuracy of measurement due to accurate receive of the mixed light by a light receptive surface of the light-receiving and computing section 302. Generally, since the surface of the thin plate has an incline, the measurement light reflected on the surface of the thin plate has an incline or a deviation with respect to the optical path to the light-receiving and computing section 302. The incline or the deviation disturbs accurate receive of the measurement light by the light receptive surface of the light-receiving and computing section 301. The convergent optical system provides secure converge of the measurement light on the light receptive surface even if the incline or the deviation is generated.

[0046] [Measuring Operation]

[0047]FIG. 3 is a view describing a method of performing a measurement of the surface displacement with respect to a whole surface of the wafer w by the optical displacement gauge 200. The wafer w is rotated in one direction in a vertical direction as shown in FIG. 1. While the optical displacement gauge 200 is being moved in a radial direction from an outer periphery A of the wafer w toward a center B, the measurement of the surface displacement is performed. Accordingly, a position of the optical displacement gauge 200 moves along a spiral line shown by a locus S, with respect to the wafer w. By performing the measurement of the surface displacement by the optical displacement gauge 200 on the locus S with a suitable interval, it is possible to efficiently execute the measurement of the surface displacement with respect to the whole surface of the wafer w. Since it is sufficient to linearly move the optical displacement gauge 200 in a horizontal direction at a distance of radius A-B, a moving mechanism of the optical displacement gauge 200 becomes simple.

[0048] [Acceleration and Reduction Process]

[0049] Although the measurement of the surface displacement of the wafer w may be performed after starting the rotation of the wafer w and the rotational speed becomes constant, a method described below is more efficient.

[0050]FIG. 4A shows a change of rotational speed (angular velocity ω) of wafer w during one measurement tact, and FIG. 4B shows linear moving speed Vx of the optical displacement gauge. When the wafer w held by the hollow spindle is rotated by a motor or the like, it is impossible to immediately achieve a predetermined rotational speed at a time of starting the rotation due to a moment of inertia of the wafer w, a rotating member of the hollow spindle and the like. The rotational speed ω is gradually increased from a state of rotational speed ω=0 so as to reach a predetermined speed after a given time. When the rotation is stopped after the measurement is finished, the rotational speed ω is gradually reduced so as to be returned to 0 after a given time has passed.

[0051] In the same manner, with respect to the linear movement of the optical displacement gauge, the speed is gradually increased from a state of speed Vx=0 when starting the movement, and the speed Vx is gradually reduced to be returned to 0 when finishing the movement.

[0052] If the measurement were performed while the rotation speed ω of the wafer w is constant, a time for acceleration before starting the measurement and a time for reduction after finishing the measurement would be required in addition to an inherent measuring time, resulting in a long measurement time. Therefore, as shown in FIG. 4, the linear movement of the optical displacement gauge is started at the same time of starting the rotation of the wafer w, and a displacement measurement by the optical displacement gauge (data acquisition) is also started.

[0053] By synchronizing the motions of the wafer w and the optical displacement gauge 200, it is possible to relatively move the wafer w and the optical displacement gauge 200 along the spiral locus S shown in FIG. 3. The synchronization is achieved by detecting the rotational speed of the wafer w and the linear moving speed or the moving position of the optical displacement gauge 200 by sensors such as a rotary encoder, a position sensor or the like, arithmetically processing the detected speeds by a computing means such as a microcomputer or the like, and controlling driving motors or the like for the wafer w and the optical displacement gauge on the basis of results of the arithmetical process. Further, the displacement measurement is executed by the optical displacement gauge at every predetermined positions set on the locus S.

[0054] The optical displacement gauge 200 starts moving from an outer peripheral position A of the wafer w. As shown in FIG. 4B, the moving speed Vx is accelerated until the optical displacement gauge reaches near middle of the radius A-B from the outer peripheral position A, and is immediately reduced when the speed reaches a given speed (for example, Vx=8 mm/sec²). The optical displacement gauge is stopped at the center portion B. As shown in FIG. 4A, the rotational speed ω of the wafer w is accelerated to its maximum value (for example, ω=240 rpm) synchronously with the moving speed Vx of the optical displacement gauge after starting the rotation until the moving speed Vx reaches its peak value, and is reduced from the time when the moving speed Vx reaches its peak. The rotation of the wafer w is stopped at the same time when the optical displacement gauge stops.

[0055] In accordance with the operation mentioned above, the measurement tact is remarkably reduced in comparison with the method of starting the measurement after the rotation speed reaches predetermined value and reducing speed to stop after the measurement is finished.

[0056] [Sensing Pin Reflector]

[0057] In the case of measuring the surface displacement of the thin plate by the optical displacement gauge, a poor reflection property of the surface of the thin plate cases an insufficient reflection of the measurement light, resulting in difficulty of measurement and inaccuracy of measurement result. Dispersion of the reflection property in accordance with the place on the surface of the thin plate easily generate dispersion of measurement results.

[0058] By providing with the sensing pin reflector, the problem caused by the reflection property of the surface of the thin plate can be solved. The sensing pin reflector is brought into contact with the surface of the thin plate, moves following to the displacement of the surface of the thin plate, and has a reflection surface reflecting the measurement light.

[0059] The sensing pin reflector generally has a contact element having a fine tip end and made of a hard material such as a diamond or the like, and a reflection surface arranged on a back surface of the contact element and finished in a mirror surface having a high reflection rate. In order to make it possible to move the sensing pin reflector in a contact and non-contact direction following to the surface of the thin plate, the structure can be made such as to support the pin portion and the reflection surface to a supporting arm constituted by a spring plate or the like and being elastically deforming in a cantilever manner, thereby making it possible to move the pin portion and the reflection surface due to an elastic deformation of the spring plate. A parallel plate-like supporting arm constituted by a pair of parallel plate pieces arranged in parallel to the surface of the thin plate and with an interval can be employed as the supporting arm. A parallelogram mechanism constituted by the pair of parallel plate pieces is a so-called parallel link mechanism structure. When deforming the parallel plate-like supporting arm supporting the contact element due to the displacement of the thin plate, the pair of plate pieces are deformed while maintaining a parallelogram keeping a parallel state. The contact element and the reflection surface supported to the front end of the parallel plate-like supporting arm move in parallel with keeping their posture. Since the posture of the contact element and the reflection surface is not changed, it is possible to reflect the measurement light in the same direction accurately, resulting in reduction of the inclination and the shift of the measurement light received by the light-receiving section.

[0060] As shown in FIGS. 5 to 7, a sensing pin reflector 60 comprises a base portion 61 mounted to the front end in the side of the wafer w of the optical displacement gauge 200, a responding portion 63 having a contact element 62 and a reflection surface 64, and a parallel plate-like supporting arm 65 connecting the sensing pin reflector 60 and the responding portion 63.

[0061] The contact shoe 62 is made of a diamond, has a thin tip end about 10 μm, and is substantially contacted with the surface of the wafer w at its tip end in accordance with a point contact. The reflection surface 64 is constituted by a mirror surface of a glass or a metal, and efficiently reflects the measurement light. The reflection surface 64 is normally opposed to an irradiating direction of the measurement light, and the tip end of the contact element 62 is arranged in an extending point of the irradiating direction of the measurement light.

[0062] As shown in FIG. 6, the parallel plate-like supporting arm 65 comprises two plate pieces 66 made of an elastically easily deforming material such as a spring plate or the like arranged in parallel with a vertical space. Two parallel plate pieces 66 are fixed each other at the base portion 61 and the responding portion 63, constituting a parallelogram link mechanism or a so-called parallel link mechanism. The plate pieces 66 is manufactured by a leaf spring material, and for example has a thickness of about 10 μm and a length of about 10 mm.

[0063] As shown in FIG. 7, an outer shape of the plate piece 66 in a plain view is formed in a trapezoidal shape which is wide in the side of the base portion 61 and narrow in the side of the responding portion 63, and a cutout portion 67 formed in one size smaller trapezoidal shape passes through a center of the trapezoidal outer shape. The rest portion of the plate piece 66 is structured such that narrow band portions 68 arranged in right and left with a space are arranged in a tapered shape which becomes wide in the side of the base portion 61 and narrow in the side of the responding portion 63.

[0064] In the sensing pin reflector 60 having the structure mentioned above, the upward displacement of the surface of the wafer w with respect to the optical displacement gauge 200 results in that the contact element 62 is pressed by the surface of the wafer w and moves upward so as to press upwardly the parallel plate-like supporting arm 65. Since the upper and lower plate pieces 66 constituting the parallelogram link mechanism mentioned above are independently deformed so as to warp to an upper side while keeping the parallelity, the responding portion 63 moves upward and downward while substantially keeping the parallel state with respect to the base portion 61. Since the reflection surface 64 provided in the responding portion 63 also moves in parallel, it moves vertically while keeping a state of normally opposing to the measurement light 1. As a result, the reflection light can be always returned in the same direction as the incident direction of the measurement light irrespective of the displacement of the surface of the wafer w.

[0065] There is a possibility that the surface displacement of the wafer w is generated in both of upward and downward directions. By lightly pressing the sensing pin reflector 60 to the surface of the wafer w previously so as to execute the measurement with keeping elastic deformation of the parallel plate-like supporting arm 65, it is possible to securely bring the contact element 62 into contact with the wafer w with respect to the surface displacement in both of the upward and downward directions. Specifically, the parallel plate-like supporting arm 65 may be arranged so as to be elastically deformed at about 100 μm.

[0066] When the direction of the reflection light of the measurement light is inclined or shifted, it becomes hard to securely receive the light by the light receptive surface of the light-receiving section as mentioned above. However, the sensing pin reflector 60 using the parallel plate-like supporting arm 65 constituting the parallelogram link mechanism mentioned above makes it hard to generate the incline or the displacement of the reflection light. Further, the parallel plate-like supporting arm 66 having the tapered structure constituted by the right and left band portions 68 effectively prevents the inclination of the responding portion 63 in the right and left directions of the tapered shape and a torsion of the parallel plate-like supporting arm 65, so that it is possible to suitably maintain the reflecting direction of the measurement light.

[0067] A function of preventing the reflection light from being inclined or shifted corresponds to a common function to the focus lens 303 mentioned above. Accordingly, if the sensing pin reflector 60 is provided, a desired function can be achieved without the focus lens. However, if the focus lens is also provided in addition to the sensing pin reflector 60, it is possible to achieve a higher function.

[0068] Further, by using the sensing pin reflector 60, it is possible to achieve a reflecting function at a higher efficiency by the reflection surface 64. Even in the case that the surface of the wafer w is made of the material having a low reflection factor or has a structure in which a reflection factor is different in accordance with places, an accurate and sable displacement measurement is achieved by reflecting the measurement light by the reflection surface 64 having high and stable reflectivity.

[0069]FIG. 8 shows a structure of an optical displacement gauge of a configuration measuring apparatus in accordance with a second embodiment of the present invention. The illustrated structure is for of measuring the thin plate principally by a spot diameter about 0.01 mm.

[0070] As shown in FIG. 8, a linear polarization laser beam having a single wavelength emitted from a light output section 301, which has a laser beam stabilized in a frequency so as to have wavelength λ as a light source, is set its polarization surface to oblique 45 degree with respect to the polarization beam splitter 11, and therefore branched into a measurement light and a reference light by the polarization beam splitter 11. Thereafter, the measurement light is injected into a lens 13 through a λ/4 wavelength plate. If the laser beam were injected to an optical center of the lens 13, it would be reflected and returned by a thin plate 4 along the same path. Thus, the laser beam is injected to a position having an offset larger than a beam diameter with respect to the optical center of the lens 13.

[0071] The measurement light branched by the polarization beam splitter is irradiated onto the thin plate 4 via the lens 13 to be reflected at a first time.

[0072] Then, the light is transmitted along the following path so that the light is again irradiated onto the thin plate 4 for an accurate measurement. The light reflected on the thin plate 4 is again transmitted through the lens 13 and the λ/4 wavelength plate 12, thereby its polarization direction is changed at 90 degrees, and the light is again injected into the polarization beam splitter 11 to be reflected in a direction different from the incident direction. The light reflected by the polarization beam splitter 11 is irradiated to a miller 15 through a λ/4 wavelength plate 14. The light reflected on the mirror 15 is again passed through the λ/4 wavelength plate 6, resulting in a rotation of the polarization surface at 90 degrees. Due to the rotation of the polarization surface, the light is transmitted through the polarization beam splitter 11. The transmitted light is irradiated on a miller 17 by a lens 16 at a focus point of the lens 16. For the same reason as in case of the lens 13, the light is injected to the lens 16 at a point having an offset larger than a beam diameter with respect to an optical center of the lens 16. The light is reflected by the miller 17, collimated by the lens 16, again transmitted through the polarization beam splitter 11, and irradiated to the miller 15 through the λ/4 wavelength plate 14. The light reflected by the miller 17 is again passed through the λ/4 wavelength plate 14 resulting in a rotation of the polarization surface at 90 degrees. Due to the change of the polarization surface, the light is reflected in a different direction by the polarization beam splitter 11. The reflected light is again irradiated to the same portion in the thin plate 4 through the λ/4 wavelength plate 12, thereby a reflection at a second time is performed.

[0073] Further, in order to introduce to a light-receiving and computing section 302 for measuring the light, the polarization direction is changed by again passing through the λ/4 wavelength plate 12 after collimating the light by the lens 13, and the light passes through the polarization beam splitter 12 to be introduced to the received-light and computing section 302 as a measurement light 18.

[0074] A reference light which is interfered with the measurement light 18 having a phase changed due to a change of the position of the thin plate 4 so as to generate an interference fringe is produced in accordance with the following procedures.

[0075] The reference light reflected in a direction of 90 degrees by the polarization beam splitter 11 so as to be branched is focused on the mirror 17 through the lens 16 so as to be reflected, and is again injected to the polarization beam splitter 11 through the lens 16. Then, the light change its direction so as to be introduced to the light-receiving and computing section 302 as a reference light 19.

[0076] The light-receiving and computing section 302 receives a mixed light of the measurement light 18 and the reference light 19, and performs a computing process for computing phase differences so as to measure displacements of the surface of the thin plate.

[0077] In this apparatus, a travel of the measurement light is changed on the basis of a difference of a distance from the optical displacement gauge to the surface of the thin plate 4, whereas a travel of the reference light is constant. Accordingly, the displacement of the surface of the thin plate can be determined by measuring a difference of travel between those of the measurement light and the reference light.

[0078] The measurement light 18 is transmitted to the thin plate surface at two times, and a displacement d of position of the thin plate surface can be accurately detected as a displacement of an optical path of 4d by the light-receiving and computing section 302.

[0079] The lens 13 which converges the output light to supply to the thin plate can be provided between the light branching and mixing section 304 and the thin plate.

[0080] The lens 13 has a function of narrowing down the measurement light irradiated to the thin plate, irradiating the measurement light only in a narrow range of the thin plate and increasing a measurement accuracy. The lens 13 functions to focus the measurement light irradiated to the thin plate 4 so that the measurement light is irradiated only to a narrow area of the thin plate 4 for increasing measurement accuracy. Further, since the distance from the lens 13 to the thin plate 4 can be set short, it is possible to make a spot diameter on the thin plate small to about 0.01 mm, resulting in accurate measurement including measurement of surface roughness of the thin plate.

[0081] Further, a convergent optical system constituted by a focus lens converging the mixed light and supplying it to the light-receiving and computing section 302 may be provided between the light branching and mixing section 304 and the light-receiving and computing section 302. The convergent optical system is constituted by the optical members such as the lens, the mirror and the like. The convergent optical system improves an accuracy of measurement due to accurate receive of the mixed light by a light receptive surface of the light-receiving and computing section 302. Generally, since the surface of the thin plate has an incline, the measurement light reflected on the surface of the thin plate has an incline or a deviation with respect to the optical path to the light-receiving and computing section 302. The incline or the deviation disturbs accurate receive of the measurement light by the light receptive surface of the light-receiving and computing section 301. The convergent optical system provides secure converge of the measurement light on the light receptive surface even if the incline or the deviation is generated.

[0082]FIG. 9 shows a structure of an optical displacement gauge of a configuration measuring apparatus in accordance with a third embodiment of the present invention. The illustrated structure is for measuring the thin plate principally by a light having a spot diameter of approximately 0.01 mm.

[0083] As shown in FIG. 9, a linear polarization laser beam having a single wavelength emitted from a light output section 301, which has a laser beam stabilized in a frequency so as to have wavelength λ as a light source, is set its polarization surface to oblique 45 degree with respect to the polarization beam splitter 11, and therefore branched into a measurement light and a reference light by the polarization beam splitter 11. Thereafter, the measurement light is injected to a lens 13 through a λ/4 wavelength plate 12. If the laser beam were injected to an center of the lens 13, it would be reflected and returned by a thin plate 4 along the same path. Thus, the laser beam is injected to a position having an offset larger than a beam diameter with respect to the optical center of the lens 1.

[0084] The measurement light branched by the polarization beam splitter is irradiated onto the thin plate 4 through a λ/4 wavelength plate 12 and the lens 13 to be reflected at a first time.

[0085] Then, the light is transmitted along the following path so that the light is again irradiated onto the thin plate 4 for an accurate measurement. The light reflected on the thin plate 4 is again transmitted through the lens 13 and λ/4 wavelength plate 12, thereby its polarization direction is changed at 90 degrees, and the light is again injected into the polarization beam splitter 11 to be reflected in a direction different from the incident direction. The transmitted light is irradiated on a miller 17 by a lens 16 at a focus point of the lens 16. For the same reason as in case of the lens 13, the light is injected to the lens 16 at a point having an offset larger than a beam diameter with respect to an optical center of the lens 16. The light is reflected on a miller 17, collimated by the lens 16, again passed through the polarization beam splitter 11, and again irradiated to the same point of the thin plate 4, thereby a reflection at a second time is performed.

[0086] Further, in order to introduce to a light-receiving and computing section 302 for measuring the light, the polarization direction is changed by again passing through the λ/4 wavelength plate 12 after collimating the light by the lens 13, and the light passes through the polarization beam splitter 12 to be introduced to the received-light and computing section 302 as a measurement light 18.

[0087] A reference light which is interfered with the measurement light 18 having a phase changed due to a change of the position of the thin plate 4 so as to generate an interference fringe is produced in accordance with the following procedures.

[0088] The reference light reflected in a direction of 90 degrees by the polarization beam splitter 11 so as to be branched is irradiated to the mirror 15 via the λ/4 wavelength plate 14. The reflected light is again transmitted through the λ/4 wavelength plate 14, thereby its polarization surface is rotated at 90 degrees. Due to the change of the polarization surface, the light is transmitted through the polarization beam splitter 11. The transmitted light is focused on the miller 17 by the lens 16, reflected by the miller 17, and again transmitted through the polarization beam splitter 11 after passing through the lens 16. The transmitted light is again irradiated to the miller 15 through the λ/4 wavelength plate 14, and again passed through the λ/4 wavelength plate 14 so as to be rotated its polarization surface at 90 degrees. Due to the change of the polarization surface, the direction of the light is changed to be introduced to the light-receiving and computing section 302 as a reference light 19.

[0089] The light-receiving and computing section 302 receives a mixed light of the measurement light 18 and the reference light 19, and performs a computing process for computing phase differences so as to measure displacements of the surface of the thin plate.

[0090] In this apparatus, a travel of the measurement light is changed on the basis of a difference of a distance from the optical displacement gauge to the surface of the thin plate 4, whereas a travel of the reference light is constant. Accordingly, the displacement of the surface of the thin plate can be determined by measuring a difference of travel between those of the measurement light and the reference light.

[0091] The measurement light 9 is transmitted to the thin plate surface at two times, and a displacement d of position of the thin plate surface can be accurately detected as a displacement of an optical path of 4d by the light-receiving and computing section 302.

[0092] The lens 13 which converges the output light to supply to the thin plate can be provided between the light branching and mixing section 304 and the thin plate.

[0093] The lens 13 has a function of narrowing down the measurement light irradiated to the thin plate, irradiating the measurement light only in a narrow range of the thin plate and increasing a measurement accuracy. The lens 13 functions to focus the measurement light irradiated to the thin plate 4 so that the measurement light is irradiated only to a narrow area of the thin plate 4 for increasing measurement accuracy. Further, since the distance from the lens 13 to the thin plate 4 can be set short, it is possible to make a spot diameter on the thin plate small to about 0.01 mm, resulting in accurate measurement including measurement of surface roughness of the thin plate.

[0094] Further, a convergent optical system constituted by a focus lens converging the mixed light and supplying it to the light-receiving and computing section 302 may be provided between the light branching and mixing section 304 and the light-receiving and computing section 302. The convergent optical system is constituted by the optical members such as the lens, the mirror and the like. The convergent optical system improves an accuracy of measurement due to accurate receive of the mixed light by a light receptive surface of the light-receiving and computing section 302. Generally, since the surface of the thin plate has an incline, the measurement light reflected on the surface of the thin plate has an incline or a deviation with respect to the optical path to the light-receiving and computing section 302. The incline or the deviation disturbs accurate receive of the measurement light by the light receptive surface of the light-receiving and computing section 301. The convergent optical system provides secure converge of the measurement light on the light receptive surface even if the incline or the deviation is generated.

[0095] In the configuration apparatus and method in accordance with the present invention, displacements on both surfaces of the thin plate are measured by the pair of optical displacement gauges. Based on the measurement results of the optical displacement gauges, thickness variation of the thin plate is measured. Further, the measurement light is irradiated to the thin plate twice. Therefore, correct and highly accurate measurement is achieved.

[0096] Although the present invention has been fully described by way of the examples with reference to the accompanying drawings, it is to be noted here that various changes and modifications will be apparent to those who skilled in the art. Therefore, unless such changes and modifications otherwise depart from the spirit and scope of the present invention, they should be construed as being therein. 

What is claimed is:
 1. A configuration measuring apparatus comprising: a light source emitting a light having a wavelength λ; a first lens to which the light emitted from the light source is injected at a position shifted from a center thereof, the first lens focusing the light on a subject to be measured; a polarization beam splitter branching the light emitted from the first lens and emitting the light to the subject; a first λ/4 wavelength plate arranged between the polarization beam splitter and the subject; a second lens to which the light reflected onthe subject through the polarization beam splitter is injected at a position shifted from a center thereof, the second lens collimating the light; a first mirror reflecting the light emitted from the second lens; a second λ/4 wavelength plate arranged between the first mirror and the second lens; a second mirror reflecting the light reflected on the first mirror through the second λ/4 wavelength plate and the polarization beam splitter at a position of a focal point thereof and reflecting the other of the branched light; a light receiving section receiving the light reflected on the second mirror through the polarization beam splitter and the first lens; and a computing section measuring a configuration of the subject on the basis of an interference signal of the received light.
 2. A configuration measuring apparatus comprising: a light source emitting a light having a wavelength λ; a polarization beam splitter branching the light emitted from the light source and emitting the light to a subject to be measured; a third λ/4 wavelength plate arranged between the polarization beam splitter and the subject; a third lends to which the light emitted from the third λ/4 wavelength plate is injected at a position shifted from a center thereof, the third lends focusing the light on the subject; a fourth lens to which the light reflected on the subject through the third lens, the third λ/4 wavelength plate, and said polarization beam splitter is injected at a position shifted from a center thereof, the fourth lend focusing the light; a third mirror reflecting the light emitted from the fourth lens at a focus point thereof; a fourth mirror reflecting the light reflected onthe third mirror through the fourth lens and the polarization beam splitter; a fourth λ/4 wavelength plate arranged between the fourth mirror and the polarization beam splitter; a light receiving section receiving the light reflected on the fourth mirror; and a computing section measuring a configuration of the subject on the basis of an interference signal of the received light.
 3. The configuration measuring apparatus according to claim 1 or 2, further comprising a focus lens 303 focusing the incident light to the light receiving section.
 4. The configuration measuring apparatus according to any one of claims 1 to 3, wherein the light source is a laser light source of a single frequency, an output light from the light source is a linear polarization light having a polarization direction inclined at 45 degrees with respect to the polarization beam splitter.
 5. The configuration measuring apparatus according to any one of claims 1 to 4, further comprising a sensing pin reflector brought into contact with a surface of the subject, moving following to a surface displacement, and having a reflection surface reflecting a measurement light.
 6. A configuration measuring method comprising: a step of branching a light; a step of focusing one of the branched light to a subject to be measured; a step of again focusing the light reflected onthe subject to the same portion of the subject; a step of making the reflection light at second time or later from the subject interfere with the other of the branched light; and a step of measuring a configuration of the subject on the basis of an signal generated by the interference.
 7. A configuration measuring method comprising: a step of focusing a light having a wavelength λ by a first lens; a step of branching the focused light by a polarization beam splitter; a step of focusing one of the branched light to a subject to be measured through a first λ/4 wavelength plate; a step of again injecting the other of the branched light to the polarization beam splitter; a step of injecting the light reflected on the subject to the polarization beam splitter through the first λ/4 wavelength plate; a step of collimating the incoming light emitted from the polarization beam splitter by a second lens; a step of passing the collimated light through a second λ/4 wavelength plate two times and injecting the collimated light to the polarization beam splitter through the second lens; a step of reflecting the incoming light emitted from the polarization beam splitter at a focal point thereof so as to inject to the polarization beam splitter; a step of collimating the incoming light emitted from the polarization beam splitter by the second lens; a step of passing the collimated light through the second λ/4 wavelength plate two times and injecting the collimated light to the polarization beam splitter through the second lens; a step of focusing the incoming light emitted from the polarization beam splitter to the same portion of the subject through the first λ/4 wavelength plate; a step of injecting the light reflected onthe subject to the polarization beam splitter through the first λ/4 wavelength plate; a step of making the incoming light emitted from the polarization beam splitter interfere with the other of the branched light and irradiating the interfered light to the right receiving section through the first lens; and a step of measuring a configuration of the subject on the basis of a signal generated by the interference.
 8. A configuration measuring method comprising: a step of branching a light having a wavelength λ by a polarization beam splitter; a step of focusing one of the branched light to a subject to be measured through a third λ/4 wavelength plate and a third lens; a step of reflecting the other of the branched light through a fourth lens at a focal point thereof and injecting the other of the branched light to the polarization beam splitter again through the fourth lens; a step of injecting the light reflected on the subject to the polarization beam splitter through the third lens and the third λ/4 wavelength plate; a step of passing the incoming light emitted from the polarization beam splitter through a fourth λ/4 wavelength plate two times and injecting again the light to the polarization beam splitter; a step of reflecting the incoming light emitted from the polarization beam splitter at a focal point thereof through the fourth lens and injecting again the light to the polarization beam splitter through the fourth lens; a step of passing the incoming light emitted from the polarization beam splitter through the fourth λ/4 wavelength plate two times and injecting again the light to the polarization beam splitter; a step of focusing the incoming light emitted from the polarization beam splitter to the same portion of the subject through the third λ/4 wavelength plate and the third lens; a step of injecting the light reflected on the subject to the polarization beam splitter through the third lens and the third λ/4 wavelength plate; a step of making the incoming light emitted from the polarization beam splitter interfere with the other of the branched light and irradiating the interfered light to the right receiving section through the first lens; and a step of measuring a configuration of the subject on the basis of a signal generated by the interference.
 9. A shape measuring method comprising: a step of branching a light having a wavelength λ by a polarization beam splitter; a step of focusing one of the branched light to a subject to be measured through a third λ/4 wavelength plate and a third lens; a step of injecting the light reflected on the subject to the polarization beam splitter through the third lens and the third λ/4 wavelength plate; a step of reflecting the incoming light emitted from the polarization beam splitter at a focal point thereof through a fourth lens and injecting again the light to the polarization beam splitter through the fourth lens; a step of focusing the incoming light emitted from the polarization beam splitter to the same portion of the subject through the third λ/4 wavelength plate and the third lens; a step of injecting the light reflected on the subject to the polarization beam splitter through the third lens and the third λ/4 wavelength plate; a step of passing the other of the branched light through the fourth λ/4 wavelength plate two times and injecting again the other of the branched light to the polarization beam splitter; a step of reflecting the incoming light emitted from the polarization beam splitter at a focal point thereof through the fourth lens and injecting again the light to the polarization beam splitter through the fourth lens; a step of passing the incoming light emitted from the polarization beam splitter through the fourth λ/4 wavelength plate two times and injecting again the light to the polarization beam splitter; a step of making the incoming light emitted from the polarization beam splitter interfere with the incoming light reflected on the same portion of the subject and irradiating the interfered light to the right receiving section; and a step of measuring a configuration of the subject on the basis of a signal generated by the interference. 